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Initially, the ability may have been thought of to give the player more recursion, returning specific cards to their hand instead of taking the risk of drawing a land in the late game, for example, but it gained more prominence as a way of putting cards in your graveyard aggressively, since, combined with draw spells and discards, the deck can use the ability to mill several cards during the turn.
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For that, the dredged card needs to be in the graveyard, and you need to pay the Dredge cost, which is to mill a specific number of cards from your own deck, thus returning the specific card to the hand.
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Dredge is an ability that allows you to return a certain card from the Graveyard to your hand, in place of any draws you would make during the turn (which may include your initial draw).
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